Apparatus for obtaining a bulk single crystal using supercritical ammonia

ABSTRACT

An apparatus includes an autoclave for preparing a supercritical solvent, a convection controller for establishing a convection flow, a dissolution zone where a feedstock is located above the convection controller and a crystallization zone where a seed is located below the convection controller are formed. A convection flow rate of the supercritical solution between the dissolution zone and the crystallization zone is determined by the degree of opening of the convection controller and the temperature difference between the dissolution zone and the crystallization zone. Accordingly, the supercritical solution, in which the nitride has a negative temperature coefficient of solubility, is supplied from the dissolution zone to the crystallization zone in which a seed is located through the convection controller so that nitride crystal is selectively grown on the seed by maintaining supersaturation of the supercritical solution with respect to the seed at a raised temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage Application under 35 USC 371of International Application No. PCT/JP02/12956, filed on Dec. 11, 2002.The disclosure of International Application No. PCT/JP02/12956 is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to an improvement of an apparatus forobtaining a nitride bulk single crystal by crystallizing fromsupercritical solution on a seed.

BACKGROUND ART

A method of obtaining a nitride bulk single crystal by recrystallizingfrom supercritical ammonia-containing solution is disclosed in JapanesePatent Application No. 2002-143449. The apparatus for obtaining anitride bulk single crystal using supercritical ammonia-containingsolution comprises an autoclave for preparing supercritical solventequipped with a convection control means for establishing a convectionflow, and mounted inside a furnace unit equipped with a heating deviceand a cooling device,

wherein the furnace unit is controlled to have a predeterminedtemperature gradient within the autoclave by the heating device and/orcooling device,

wherein the convection control means for establishing convection flowcomprises at least one horizontal baffle having a central opening and/ora space between the baffle and an inner wall of the autoclave, andseparating the dissolution zone where a feedstock is located above thebaffle from the crystallization zone where a seed is located below thebaffle.

Furthermore, it is found that a convection flow rate of thesupercritical solution between the dissolution zone and thecrystallization zone may be determined by a degree of opening of theconvection control means and a temperature difference between thedissolution zone and crystallization zone etc.

The result of the inventors' research shows that a growth rate of 10μm/h or more is required for commercially manufacturing of nitride. Itis also found to be necessary that nitride is dissolved in thesupercritical solvent containing ammonia and at least alkali metal ionsto make the supercritical solution, in which the nitride has a negativetemperature coefficient of solubility and the supercritical solution issupplied from the dissolution zone to the crystallization zone where aseed is located through the convection control means so that nitridecrystal can be selectively grown on the seed arranged in the autoclaveby maintaining supersaturation of the supercritical solution withrespect to the seed at the predetermined raised temperature andcontrolling below a certain concentration so as not to allow spontaneouscrystallization.

DISCLOSURE OF INVENTION

An object of the present invention is, therefore, to provide anautoclave in which a convection flow rate can be controlled for growinga nitride bulk single crystal.

Another object of the present invention is to provide an autoclave inwhich contamination by impurities from the autoclave outer wall can beprevented.

Furthermore, the other object of the present invention is to provide anautoclave in which a commercially valuable growth rate can be attained.

These goals can be achieved by the invention based on a technique of anammono-basic growth of a crystal which comprises a chemical transport ina supercritical ammonia-containing solvent containing at least onemineralizer for imparting an ammono-basic property, to grow a nitridesingle crystal;

The apparatus comprises an autoclave 1 for preparing supercriticalsolvent equipped with a convection control means 2 for establishing aconvection flow, and mounted inside a furnace unit 4 equipped with aheating device 5 and a cooling device 6,

wherein the furnace unit 4 is controlled to maintain a predeterminedtemperature gradient within the autoclave by the heating device 5 and/orcooling device 6,

wherein the convection control means 2 comprises at least one horizontalbaffle 12 having a central opening and/or a space between the baffle andan inner wall of the autoclave, and separating the dissolution zone 13where a feedstock 16 is located above the baffle from thecrystallization zone where a seed 17 is located below the baffle,

wherein a convection flow rate of the supercritical solution between thedissolution zone 13 and the crystallization zone 14 is determined by adegree of opening of the convection control means 2 and a temperaturedifference between the dissolution zone 13 and crystallization zone 14,

wherein nitride is dissolved in the supercritical solvent containingammonia and at least alkali metal ions to make the supercriticalsolution, in which the nitride has a negative temperature coefficient ofsolubility and the supercritical solution is supplied from thedissolution zone 13 to the crystallization zone 14 in which a seed islocated through the convection control means 2, so that nitride crystalis selectively grown on the seed arranged in the autoclave bymaintaining supersaturation of the supercritical solution with respectto the seed at the predetermined raised temperature and controllingbelow a certain concentration so as not to allow spontaneouscrystallization.

In the technique of an ammono-basic growth of a crystal, it is foundthat the crystal growth may be influenced by the composition andconcentration of the supercritical ammonia-containing solution, thetemperature difference between the dissolution zone and thecrystallization zone, the location and area of the baffle by which aconvection flow rate is controlled basing on the temperature difference,the filling ratio of ammonia and the ratio of surface area of the seedwith respect to that of the feedstock etc. Furthermore, according to thepresent invention, a convection flow rate of the supercritical solutionbetween dissolution zone 13 and crystallization zone 14 can bedetermined by the convection control means 2 and the temperaturedifference, therefore nitride crystal can be selectively grown on theseed by maintaining supersaturation of the supercritical solution withrespect to the seed and controlling below a certain concentration so asnot to allow spontaneous crystallization.

Moreover, supercritical ammonia-containing solution containing alkalimetal ions has an excellent solubility, thereby it is possible todecrease contamination by impurities from the autoclave inner wall if alining of metal such as Ag, Mo, Fe or Ta, or alloy thereof is applied toan inner wall of the autoclave.

The convection control means is used to create a temperature gradientdifference between the dissolution zone and the crystallization zone,and the form and area of the convection control means can be varied bythe volume of the autoclave and the ratio of inside diameter to thetotal length of the autoclave. It is preferable that the convectioncontrol means may be formed within the range of 70% to 90% of the innercross-sectional area of the autoclave and the ratio of opening in bafflemay be set at 30% or less. The location of baffle may be adjusted inaccordance with the quantity of the grown crystal and the baffle may belocated within the range from 1/3to 2/3 of the total length of the innerautoclave, thereby to adjust the ratio between the dissolution zone andthe crystallization zone to a desired value. It is preferable that thefeedstock is placed in the dissolution zone and the filling ratio of thefeedstock is less than half of the dissolution zone. In case that thefeedstock is in the form of gallium metal, the filling ratio of thefeedstock may be about 1/4 of the dissolution zone because the volume ofthe feedstock will be increased by transforming from gallium metal topoly-GaN in the crucible.

In the area of the convection control means 2, it is preferable that thecooling device 6 is arranged for cooling so that it is easier to make apredetermined temperature difference between the dissolution zone 13 andcrystallization 14. It is also preferable that the cooling device 18which can cool the bottom of the flowing area of the crystallizationzone is placed in the autoclave and thereby rapid cooling function isexecuted after crystallization.

Using above constitution of the autoclave, it is possible to improve thegrowth rate on a seed. It is preferable that the ratio of diameter tototal length of the autoclave may be set from 1/15 to 7/15, the ratio ofopening in the horizontal baffle on the cross-sectional area may be setat 30% or less and growth rate on a seed may be 10 μm/hr or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a relation between pressure and GaN solubility in thesupercritical ammonia containing potassium amide (at molar ratio ofKNH₂:NH₃=0, 07) at T=400° C. and T=500° C.

FIG. 2 shows a graph illustrating a relation of change in temperature inan autoclave with the passage of time and change between a dissolutionstep and a crystallization step, where the pressure is kept constant.

FIG. 3 shows a graph illustrating a relation of change in pressure in anautoclave with the passage of time and change between a dissolution stepand a crystallization step, where the temperature is kept constant.

FIG. 4 is a schematic cross-sectional view of an autoclave and a furnaceunit used in this invention.

FIG. 5 is a schematic perspective view of an apparatus for obtaining anitride bulk single crystal.

FIG. 6 shows a graph illustrating change in temperature in an autoclavewith the passage of time and a relation of change in a dissolution stepand a crystallization step in Example.

BEST MODE FOR CARRYING OUT THE INVENTION

The apparatus for obtaining a bulk single crystal comprises an autoclave1 for preparing a supercritical solvent equipped with the convectioncontrol means 2 for establishing a convection flow, and at least onefurnace unit 4 equipped with a heating device 5 and a cooling device 6on the autoclave. The furnace unit includes a higher temperature zone 14equipped with a heating device 4, which corresponds to a crystallizationzone in the autoclave, and a lower temperature zone 13 equipped with aheating device which corresponds to a dissolution zone in the autoclave.It is also possible to use a furnace unit which includes a highertemperature zone equipped with a heating device and/or cooling device,and a lower temperature zone equipped with a heating device and/orcooling device. The convection control means for establishing aconvection flow is composed of at least one horizontal baffle having acentral opening and/or a periphery space so as to devide thecrystallization zone from the dissolution zone. Thus, the feedstock isset in the dissolution zone, and the seed is set in the crystallizationzone, in the autoclave. The convection flow of the supercriticalsolution between the dissolution zone and the crystallization zone iscontrolled by the convection control means. The dissolution zone islocated above the horizontal baffle, and the crystallization zone islocated below the horizontal baffle.

The apparatus for obtaining a bulk single crystal according to thepresent invention is illustrated in FIG. 4 and FIG. 5. The main part ofthe apparatus comprises the autoclave 1 used for preparing asupercritical solvent and the convection control means 2 which enhanceschemical transport in the supercritical solution within the autoclave 1.The autoclave 1 is situated in the chamber 3 of the furnace unit 4 (2sets), equipped with the heating device 5 and the cooling device 6.Position of the autoclave 1 within the furnace unit 4 is secured by ascrew blocking device 7. The furnace 4 is embedded in the bed 8 andsecured with steel tapes 9 tightly wound around the furnace unit 4 andthe bed 8. The bed 8 with the furnace unit 4 is pivotally mounted on theturntable 10 and secured in the desired position by means of a pinsecuring device 11, so that the convective form and convection flow inthe autoclave can be controlled. The convection flow of thesupercritical solution in the autoclave 1 placed in the furnace unit 4is established by means of the convection control means 2, which iscomposed of the horizontal baffle 12 of a size corresponding to about70% of horizontal cross-sectional area of the autoclave 1, dividing thecrystallization zone from the dissolution zone. The horizontal baffle 12is located approximately in the middle of the autoclave 1 in terms oflongitudinal dimension. Temperature values in each zone of the autoclave1, falling within the range from 100° C. to 800° C., are controlled by acontrol unit 15 placed near the furnace unit 4. In the autoclave 1 thedissolution zone 13 corresponding to the lower temperature zone of thefurnace unit 4 is situated above the horizontal baffle 12. The feedstock16 is placed in the dissolution zone 13 and the amount of the feedstock16 is such that its volume does not exceed ½ of volume of thedissolution zone. Simultaneously, when metallic gallium is introduced asa feedstock in crucible, the total volume of the crucible should notexceed ½ of volume of the dissolution zone. The crystallization zone 14corresponding to higher temperature zone of the furnace unit 4 issituated below the horizontal baffle 12. In the crystallization zone 14the seed 17 is located and the specific position in which the seed 17 isplaced is below crossing of up-stream convection flow and down-streamconvection flow, but still above the bottom of the crystallization zone.The cooling device 6-1 for cooling is placed within the zone of theconvection control means 2. As the result, the predetermined temperaturedifference between the dissolution zone 13 and the crystallization zone14 may be controlled. At the level of the bottom of the crystallizationzone there is another cooling device 6-2, used in order to cool down thezone after the crystallizing process is over, so that the dissolution ofthe grown crystal during the cooling stage after the crystallizingprocess can be remarkably prevented.

According to the result of the research, GaN exhibits good solubility inNH₃ including alkali metals or their compounds, such as KNH₂. The graphin FIG. 1 presents how solubility of GaN in supercritical solventdepends on the pressure, for temperature 400° C. and 500° C. Here thesolubility is defined as the molar percentage: S_(m)≡[GaN^(solution):(KNH₂+NH₃)]×100%. In this case, the solvent is the supercriticalammonia-containing solution in which the molar ratio X≡KNH₂: NH₃ is setat 0.07. Solubility S_(m) may be a function of three parameters:temperature, pressure, and molar ratio of the mineralizer, i.e.S_(m)≡S_(m) (T, p, x). Small changes of ΔS_(m) can be expressed as:ΔS_(m)≈(∂S_(m)/∂T)_(p,x ΔT+(∂S) _(m)/∂p)_(T,x ΔP+(∂S) _(m)/∂x)_(T,x)Δx,where the parameters in the partial derivatives, e.g. each coefficientof (∂S_(m)/∂T)_(p,x) and so on, is defined as a temperature coefficientof the solubility and a pressure of the solubility, and a molar ratiocoefficient of the mineralizer.

As it results form the above graph presented in FIG. 1, the solubilityis an increasing function of pressure and a decreasing function oftemperature. On the basis of these dependences it is possible to obtainGaN bulk single crystal by dissolving it under higher solubilityconditions and crystallizing under lower solubility conditions. Negativetemperature coefficient of solubility means that in the presence of atemperature gradient the chemical transport of nitride occurs from thelower temperature dissolution zone to the higher temperaturecrystallization zone. Furthermore, it is also found that other galliumcompounds and metallic gallium can be used as a supplier for GaNcomplex.

For example, Ga complex, such as metallic gallium which is the simplestelement can be introduced into the above solvent. Next, thesupersaturation of solution with respect to nitride is obtained bychange of physical conditions such as heating, so that crystal can begrown on a seed. According to the present invention, it is possible tocrystallize the desired nitride bulk single crystal on a seed and alsoto lead the stoichiometric growth of GaN obtained as a bulk singlecrystal layer on a seed in the form of GaN crystal.

The obtained single crystal may contain alkali metals at theconcentration of 0.1 ppm or more since the single crystal is grown inthe supercritical ammonia-containing solution containing alkali metalions. In view of maintaining the desired ammonobasic character of thesupercritical solution, and also in order to avoid corrosion of theapparatus, no halogens are intentionally introduced into thesupercritical solvent. According to the present invention, intentionalreplacing of 0.05 to 0.5 Ga by Al or In may be achieved. The crystallattice constants of the obtained nitride can be adjusted by aappropriate modification of the composition. GaN bulk single crystalobtained by the process according to the present invention may be alsointentionally doped with donor dopants (such as Si, O etc.), acceptordopants (such as Mg, Zn etc.) or magnetic dopants (such as Mn, Cr etc.)in concentrations of 10¹⁷ to 10²¹/cm³. The dopants may change optical,electrical and magnetic properties of nitride. As for other physicalproperties, the grown GaN bulk single crystal has a surface dislocationdensity of 10⁶/cm² or less, preferably 10⁵/cm² or less, more preferably10⁴/cm² or less, and also has the full width at half maximum of theX-ray from the surface (0002) plane of 600 arcsec. or less., preferably300 arcsec. or less, more preferably 60 arcsec. or less. It is possibleto grow a GaN bulk single crystal as the best which has a surfacedislocation density of about 10⁴/cm² or less, and the full width at halfmaximum of the X-ray from the surface (0002) of 60 arcsec. or less.

(Temperature difference between the crystallization zone and thedissolution zone)

When two zones, i.e., the dissolution zone and the crystallization zoneare simultaneously formed in the autoclave, supersaturation of thesupercritical solution with respect to the seed is maintained bycontrolling the dissolution temperature and the crystallizationtemperature. The control is found to be easy by setting the temperatureof the crystallization zone at 400 to 600° C., and by maintaining thedifference in temperature between the dissolution zone and thecrystallization zone within the autoclave, at 150° C. or less,preferably 100° C. or less. Preferably, the adjustment of thesupersaturation of the supercritical solution with respect to the seedis made by providing at least one baffle for dividing the internal ofthe autoclave into a lower temperature dissolution zone and a highertemperature crystallization zone, and controlling the convection flowbetween the dissolution zone and the crystallization zone. Further, whentwo zones, i.e., a dissolution zone and a crystallization zone, betweenwhich a specified temperature difference is set, are formed in theautoclave, the supersaturation of the supercritical solution withrespect to the seed is preferably adjusted by using angallium-containing feedstock composed of an GaN crystal having a totalsurface area larger than the total surface area of the seed.

The present invention relates to a technique of an ammono-basic growthof a crystal which comprises the steps of causing a chemical transportin a supercritical ammonia-containing solvent containing at least onemineralizer for imparting an ammono-basic property, thereby growing anitride single crystal. The terms herein should be understood as havingthe meanings defined as below in the present specification.

(Nitride)

The term “nitride” in the specification means a compound which includesat least nitrogen atom as a consistent element, defined as the generalformula Al_(x)Ga_(1−x−y)In_(y)N, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1, andmay contain at least the binary compound such as GaN or AlN, ternarycompounds such as AlGaN, InGaN or also quaternary compounds AlInGaN. Itis preferable that Al_(x)Ga_(1−x)N, where 0≦x≦1, is used and it maycontain a donor, an acceptor, or a magnetic dopant, as required. Theterm “nitride bulk single crystal” means a nitride single crystalsubstrate on which an optical and electronic device such as LED or LDcan be formed by an epitaxial growing process, such as MOCVD, HVPE orthe like.

(Precursor)

The term “a precursor of nitride ” means a substance which may containat least gallium or aluminum, and if needed, an alkali metal, an elementof the Group XIII, nitrogen and/or hydrogen, or a mixture thereof, andexamples of such a precursor include metallic Ga or Al, an alloy or anintermetallic compound of Ga or Al, and a hydride, amide, imide,amido-imide or azide of Ga or Al, which can form a gallium compound oraluminum compound soluble in a supercritical ammonia-containing solventas defined below.

(Feedstock)

The term “feedstock” means a gallium-containing nitride,aluminum-containing nitride or a precursor thereof. The process of thepresent invention is based on an ammono-basic reaction. The feedstockmay be GaN or AlN formed by HVPE, or GaN or AlN formed by chemicalreactions, which originally may contain chlorine, in so far as theammono-basic supercritical reaction is not hindered. A combination ofnitride which is dissolved in an equilibrium reaction to thesupercritical ammonia-containing solvent and metallic gallium ormetallic aluminum which irreversibly reacts with the supercriticalammonia-containing solvent can be used as a feedstock.

The control of the reaction for crystallization becomes easy by makinguse of gallium nitride as the nitride. In this case, GaN single crystalis preferably used as a seed. GaN obtained by flux method orpolycrystalline gallium nitride poly-crystallized from the supercriticalammonia of metallic Ga can be used as a feedstock for GaN.

(Supercritical ammonia-containing solvent)

In the present invention the supercritical solvent may contain NH₃ or aderivative thereof. The mineralizer may contain alkali metal ions, atleast, ions of sodium or potassium. The term “supercriticalammonia-containing solvent” means a supercritical solvent which maycontain at least ammonia, and ion or ions of at least one alkali metalfor dissolving nitride.

(Mineralizer)

The term “mineralizer” means a supplier for supplying one or more ofalkali metal ions for dissolving nitride in the supercriticalammonia-containing solvent. Mineralizer is introduced in the form ofalkali metal compound for imparting alkali metal or alkali metal ions inthe supercritical ammonia-containing solvent. In the view of the purity,it is preferable that the mineralizer in the form of alkali metallicazide (NaN₃, KN₃, LiN₃) or alkali metal (Na, K, Li) may be introduced,however, alkali metal amide can be simultaneously used, as required. Theconcentration of alkali metal ions in the supercritical solvent isadjusted so as to ensure the specified solubilities of feedstock ornitride, and the molar ratio of the alkali metal ions to othercomponents of the resultant supercritical solution is controlled withina range from 1:200 to 1:2, preferably from 1:100 to 1:5, more preferablyfrom 1:20 to 1:8. In case of using the combination of two or moreelements of alkali metal ions, the rate of crystal growth and crystalquality can be improved more, compared with the case where only oneelement of alkali metal ion is used. Moreover an alkaline earth metal,such as Mg, Zn or Cd, can be simultaneously used, as required.Furthermore, neutral element (alkali metal halide), acidic element(ammonium halide) may be simultaneously used, in so far as theammono-basic supercritical reaction is not hindered.

(Dissolution of feedstock)

Dissolution of feedstock is a reversible or irreversible process wherethe feedstock is in the form of nitride compound soluble in thesupercritical solvent, for example gallium complex compound or aluminumcomplex compound. Gallium complex compound is chemical complex compound,in which a centrally placed gallium atom is surrounded by NH₃ typeligands or their derivatives, such as NH₂ ⁻, NH²⁻.

(Supercritical ammonia-containing solution)

The term “supercritical ammonia-containing solution” means a solublegallium or aluminum compounds formed by the dissolution of the feedstockin the supercritical ammonia-containing solvent. Based on ourexperiment, we have found that there is an equilibrium relationshipbetween the metallic nitride solid and the supercritical solution undersufficiently high temperature and pressure conditions. Accordingly, thesolubility of the soluble nitride can be defined as the equilibriumconcentration of the above soluble gallium or aluminum compounds in thepresence of solid nitride. In such a process, it is possible to shiftthis equilibrium by changing temperature and/or pressure.

(Solubility)

The phrase “negative temperature coefficient of solubility” means thatthe solubility is expressed by a monotonically decreasing function ofthe temperature, when all other parameters are kept constant. Similarly,the phrase “positive pressure coefficient of solubility” means that thesolubility is expressed by a monotonically increasing function of thepressure, when all other parameters are kept constant. Based on ourresearch, the solubility of nitride in the supercriticalammonia-containing solvent, has a negative temperature coefficientwithin a range of 300 to 550° C., and a positive pressure coefficientwithin the range of 1 to 5.5 kbar. For example, as shown in FIG. 2,after dissolution of feedstock in an autoclave kept for 8 days at thetemperature 400° C. (i.e. after dissolution step), crystallization ofgallium nitride may be achieved by increasing the temperature inside theautoclave to 500° C. (crystallization step). On the other hand, as shownin FIG. 3, after dissolution of a feedstock at increased pressure in anautoclave kept for 2 days at the level of 3.5 kbar (i.e. afterdissolution step), crystallization of gallium nitride is achieved bymeans of reducing the pressure to 2 kbar in the autoclave(crystallization step).

(Supersaturation)

The phrase “supersaturation of the supercritical ammonia-containingsolution with respect to the nitride” means that the concentration ofthe soluble gallium or aluminum compounds in the above supercriticalammonia-containing solution is higher than the concentration in theequilibrium state, i.e., the solubility of nitride. In case of thedissolution of nitride in a closed system, such supersaturation can beachieved, according to the negative temperature coefficient or thepositive pressure coefficient of solubility, by raising the temperatureor reducing the pressure.

(Chemical transport)

The phrase “the chemical transport of nitride in the supercriticalammonia-containing solution” means a sequential process including thedissolution of the feedstock, the transfer of the soluble nitridethrough the supercritical ammonia-containing solution, and thecrystallization of nitride from the supersaturated supercriticalammonia-containing solution. In general, a chemical transport process iscarried out by a certain driving force such as a temperature gradient, apressure gradient, a concentration gradient, difference in chemical orphysical properties between the dissolved feedstock and the crystallizedproduct, or the like. Preferably, the chemical transport in the processof the present invention is achieved by carrying out the dissolutionstep and the crystallization step in separate zones, provided that thetemperature of the crystallization zone is maintained higher than thatof the dissolution zone so that the nitride bulk single crystal can beobtained by the processes of this invention.

(Seed)

The term “seed” has been described by way of examples in the presentspecification. The seed provides a region or area on which thecrystallization of nitride is allowed to take place and the growthquality of the crystal depends on the quality of the seed. Thus, theseed of high quality should be selected. The dislocation density thereofis preferably 10⁵/cm² or less. As a seed, a natural seed obtained fromflux method or high pressure method, A-plane, M-plane or R-plane seedobtained by bulk single crystal can also be used. Moreover, a seedhaving a seed surface exhibiting n-type electrical conductivity dopedwith Si may be used. Such seed can be produced using processes fornitride crystal growth from gaseous phase, such as HVPE or MOCVD, etc.Doping with Si during the growth process at the level of 10¹⁶ to10²¹/cm² ensures n-type electric conductivity. Moreover, a compositeseed obtained by growing AlN or GaN deposited on the electric conductivesubstrate of SiC, etc. may be used.

(Spontaneous crystallization)

The term “spontaneous crystallization” means an undesirable phenomenonin which the formation and the growth of the core of nitride from thesupersaturated supercritical ammonia-containing solution occur at anysite inside the autoclave, and the spontaneous crystallization alsoincludes disoriented growth of the crystal on the surface of the seed.

(Selective crystallization)

The term “selective crystallization on the seed” means a step ofallowing the crystallization to take place on the surface of the seed,accompanied by substantially no spontaneous growth. This selectivecrystallization on the seed is essential for the growth of a bulk singlecrystal, and it is one of the elements of the present invention.

(Feedstock)

Pellets to be used in the present invention are prepared by molding thepowder and baking them so that its density is 70% or more. Higherdensity is preferable.

(Temperature and pressure of the reaction)

The temperature distribution in the autoclave as will be described laterin the part of Example is determined by using an empty autoclave inside,i.e. without the supercritical ammonia, and thus, the supercriticaltemperature is not the one actually measured. On the other hand, thepressure in the autoclave is directly measured, or it is determined bythe calculation from the amount of ammonia introduced initially, thetemperature and the volume of the autoclave.

(Example)

High-pressure autoclave 1 (FIG. 9), having the inner diameter of 40 mm,length equal to 480 mm (D/L=1/12) and volume of 585 cm³, is charged with30 g of feedstock in the form of GaN in the crucible in the dissolutionzone 13, and GaN seed of the diameter of 1 inchΦ obtained by HVPE methodis placed in the crystallization zone 14 of the same autoclave. Next theautoclave 1 is filled with 1.2 g of 6N purity metallic gallium, 23 g of3N purity metallic sodium as a mineralizer and 238 g of ammonia (5N) andthen closed. The autoclave 1 is introduced into the furnace unit 4 andheated to 200° C. for three days.

Then the temperature in the dissolution zone 13 of the autoclave isincreased to 425° C., while the temperature in the crystallization zone14 is increased to 525° C. The resultant pressure within the autoclaveis 2.5 kbar. The autoclave is left under such condition for anothertwenty eight days. (FIG. 6) As a result of the processes, partialdissolution of the feedstock in the dissolution zone 13 and growth ofgallium nitride on the HVPE-GaN seed in the crystallization zone 14 areobserved. The total thickness of the both sides of single crystal layeris about 3 mm.

The processes described below are carried out so as to use the resultantcrystal as a substrate.

1) A single crystal layer of 3 mm thick deposited on a HVPE-GaN seed isput into a furnace and annealed for 1 to 5 hours in the nitrogenatmosphere, containing low amount of oxygen, at temperature from 600° C.to 900° C.

2) The sample is placed into the wire saw manufactured by Takatori Corp.The sample is positioned with the off-angle about 1 degree or less. Thenthe sample is sliced by diamond slurry, so that 5 slices with theoff-angle between 0.05 and 0.2 degree are obtained.

3) The sliced samples are put once more into a furnace and annealedagain for 1 to 5 hours in the nitrogen atmosphere, containing low amountof oxygen, at temperature from 600° C. to 900° C. (The thus preparedsample is called GaN substrate.)

4) The GaN substrate is adhered by adhesive agent on the block forpolishing, the block is placed on a polishing machine manufactured byLogitech Ltd. and the GaN substrate is polished consecutively on eachside. In the polishing process, diamond slurry and colloidal silica withpH from 3 to 6 or alumina solution with pH from 9 to 11 are used. Theroughness of the obtained surface is 10 Å or less.

5) Next, a cap layer of less than several μm in the form of GaN or AlGaNis formed on the surface of GaN substrate by HVPE method, under thecondition as follows, so that a template is obtained.

6) In the next step, on a GaN substrate with the above mentioned caplayer or on a GaN substrate without the cap layer, another GaN layer of3 mm thick is formed by HVPE method. After cutting and polishingaccording to the aforementioned procedures the template having thethickness about 0.5 mm, suitable for light emitting devices, isobtained.

HVPE condition: reaction temperature: 1050° C.,

reaction pressure: atmospheric pressure,

partial pressure of ammonia: 0.3 atm,

partial pressure of GaCl: 1×10⁻³ atm

hydrogen carrier gas

As required, 7) After polishing, the GaN substrate is kept in thesupercritical ammonia without mineralizer for 1 day at 200° C. and thenthe impurity on the surface of the GaN substrate is removed.

INDUSTRIAL APPLICABILITY

High quality nitride bulk single crystal can be obtained from asupercritical ammonia-containing solution.

1. An apparatus for obtaining a bulk single crystal comprising anautoclave for preparing a supercritical solvent equipped with aconvection control means for establishing a convection flow, and mountedinside a furnace unit equipped with a heating device and a coolingdevice, wherein the furnace unit is controlled to obtain a temperaturegradient within said autoclave by said heating device and/or coolingdevice, wherein the convection control means comprises at least onehorizontal baffle having a central opening and/or a space between thebaffle and an inner wall of the autoclave, and separating thedissolution zone where a feedstock is located above said baffle fromsaid crystallization zone where a seed is located below said baffle,wherein nitride is dissolved in the supercritical solvent containingammonia and at least alkali metal ions to make the supercriticalsolution, in which the nitride has a negative temperature coefficient ofsolubility and the supercritical solution is supplied from saiddissolution zone to said crystallization zone in which a seed is locatedthrough said convection control means so that nitride crystal is grownon the seed arranged in the autoclave, wherein a lining of metal such asAg, Mo, Fe or Ta, or alloy thereof is applied to an inner wall of theautoclave.
 2. The apparatus for obtaining a bulk single crystalaccording to claim 1, wherein the cooling device is placed so as to coolan area where said convection control means is located, thereby toprovide the temperature difference between said dissolution zone andsaid crystallization zone.
 3. The apparatus for obtaining a bulk singlecrystal according to claim 1, wherein the cooling device which can coolthe bottom of the flowing area of said crystallization zone is placed inthe autoclave and thereby rapid cooling function is operated aftercrystallization.
 4. The autoclave according to claim 1, wherein saidconvection control means is located within the range from ⅓ to ⅔ of thetotal length of the inner autoclave in a longitudinal direction, tothereby adjust the ratio between said dissolution zone and saidcrystallization zone to a desirable value.
 5. The autoclave according toclaim 1, wherein the ratio of a diameter of the autoclave in a directionperpendicular to a longitudinal direction relative to a total length ofthe autoclave is set from 1/15 to 7/15, the ratio of opening in saidhorizontal baffle on the cross-sectional area is set at 30% or less anda growth rate on a seed is 10 μm/hr or more.
 6. The autoclave forobtaining a bulk single crystal according to claim 1, wherein aconvection flow rate of the supercritical solution between saiddissolution zone and said crystallization zone is determined by a degreeof opening of said convection control means and a temperature differencebetween said dissolution zone and said crystallization zone.
 7. Anapparatus for obtaining a bulk single crystal comprising an autoclavefor preparing a supercritical solvent equipped with a convection controlmeans for establishing a convection flow, and mounted inside a furnaceunit equipped with a heating device and a cooling device, wherein thefurnace unit is controlled to obtain a temperature gradient within saidautoclave by said heating device and/or cooling device, wherein theconvection control means comprises at least one horizontal baffle havinga central opening and/or a space between the baffle and an inner wall ofthe autoclave, and separating the dissolution zone where a feedstock islocated above said baffle from said crystallization zone where a seed islocated below said baffle, wherein nitride is dissolved in thesupercritical solvent containing ammonia and at least alkali metal ionsto make the supercritical solution, in which the nitride has a negativetemperature coefficient of solubility and the supercritical solution issupplied from said dissolution zone to said crystallization zone inwhich a seed is located through said convection control means so thatnitride crystal is grown on the seed arranged in the autoclave, whereinthe feedstock made of gallium metal is placed in said dissolution zoneand the filling ratio of the feedstock is less than half of saiddissolution zone.
 8. The autoclave for obtaining a bulk single crystalaccording to claim 7, wherein a convection flow rate of thesupercritical solution between said dissolution zone and saidcrystallization zone is determined by a degree of opening of saidconvection control means and a temperature difference between saiddissolution zone and said crystallization zone.